1. Field of the Invention
This invention relates to image sensors. In particular the invention relates to charge coupled device (CCD) frame transfer image sensors.
2. Description of Related Art
In CCD frame transfer image sensors, light representing an image is focussed onto a first photosensitive portion of an array of CCD channels comprising an image section of the array. A second portion of the array of CCD channels which is non photosensitive, e.g. due to being shielded from light, comprises a store section. Adjacent to the store section there is provided a line read-out section. A frame of charge photogenerated in the image section is transferred to the store section by applying clock pulses to the control electrodes of both sections and then read out in sequential line-by-line fashion by applying clock pulses to the control electrodes of both store and line read-out sections to form an image on for example, a television screen. Normally a frame of charge is read out of the store section as a further frame is being collected in the image section, and so on. Thus the `integration period` during which a frame of photo-generated charge is collected in the image section of the sensor can not normally be shorter than the time taken to read out the previous frame of charge from the store section. The read out time is often fixed to conform to a given output standard, for example in conventional 525/625 line television, and is, in any case, limited by the physical constraints of the device structure, for example the speed of charge transfer. This then precludes the use of the very short integration periods which may be a desirable feature in such applications as the imaging of fast moving objects, or in high illumination situations, unless a mechanical shutter, with all its attendant problems of mechanical unreliability, is employed in front of the sensor.
In U.S. Pat. No. 3,931,463 there is described a CCD frame transfer image sensor for a television wherein the effective integration period can be shortened by electrical means.
The CCD frame transfer image sensor described in that patent comprises: an image section comprising a photo-sensitive portion of an array of CCD channels, a store section comprising a non photo-sensitive portion of said array of CCD channels; a line read-out section adjacent to the store section with a control electrode structure being associated with said image, store, and read out sections. A charge sink structure is provided at the end of said CCD channels remote from the store section of the sensor. Means are provided for applying potentials to the control electrode structure so as to transfer charge generated in the image section along the channels in either direction, i.e. either into the store section to be read out via said read-out section, or into the charge sink structure thereby to control the period for which charge is collected in the image section before transfer to the store section.
In the sensor described in U.S. Pat. No. 3,931,463 reverse clocking pulses effective to transfer charge into the charge sink structure are applied during the horizontal retrace periods of the television. This avoids spurious patterning of the picture obtained from the output signal due to parasitic pickup of the reverse clocking waveform in the output signal. Such a technique suffers the disadvantage however that it is often not possible to apply sufficient reverse clocking pulses during each horizontal retrace period to totally transfer the required charge to the charge sink. Thus as in each horizontal retrace period any photocharge collected in the preceding line time is moved towards the charge sink where there are insufficient reverse clocking pulses applied a residue of photocharge will be left, displaced vertically from its original position. After successive line periods, as further photocharge is superimposed on the displaced residual charge from the preceding reverse clocking, a pattern of multiple images is created separated by a number of CCD lines equivalent to the number of reverse clocking pulses applied during each horizontal retrace period. Whilst for some applications the horizontal retrace period is long enough to allow all photocharge in the image section to be dumped such that multiple images will not occur, for many applications such as 525/625 line TV this is not possible, and the formation of multiple images is a major disadvantage.